Improvements in capacitor design have been hampered in recent years because of the limitations in both physical and electrical properties of currently available dielectric films. These limitations reduce the thermal range, energy storage capabilities and reliability of capacitors incorporating such dielectrics, imposing particularly acute restrictions on the design and construction of high temperature, high energy density, and high repetition rate capacitors.
Polymeric materials that are used in current designs or have been tested in prototype designs include: polypropylene, PPS, PVDF, polycarbonate, polysulfone, polystyrene, Kapton, Upilex, and several other proprietary advanced polyimides. None of these materials are capable of functioning in the range of 300.degree. C. for extended periods of time without dramatic reductions in electrical integrity. Nor are they capable of meeting the energy density goals of high energy storage capacitors.
For high energy capacitor applications, the storage capacity of a given capacitor is determined primarily by the dielectric constant and the dielectric breakdown strength of the dielectric. The energy that can be stored in a capacitor comprising a dielectric medium placed between two electrodes can be represented by the equation: EQU E=1/2 CV.sup.2
where C is the capacitance of the capacitor and V is the charging voltage. For a simple parallel plate capacitor, C is given by: EQU C=K.sub.o A/t
where K.sub.o is the permittivity in free space, K is the relative permittivity, A is the area, and t is the thickness of the dielectric medium, respectively. The term dielectric constant is used instead of relative permittivity in the industry and will also be used here.
Polymers in general have relatively low dielectric constants (2 to 15), but high capacitance values can be obtained by winding thin polymer films. The energy density of a capacitor, in which most of the weight is in the polymer component, can be represented by the equation: EQU Ed=E/M=(K.sub.o) F.sup.2 /2.rho.
where F is the electric field (volts/unit thickness) and .rho. is the density of the dielectric. For practical capacitors, the energy density will be lower due to the weight of electrodes, end tabs, and the casing. But for very large capacitors, especially when metallized films are used as electrodes, this equation is a very good estimate of the energy density.
The storage capacity of a capacitor is limited by the highest electric field that can be imposed on it. This in turn is limited by the ultimate breakdown strength of the dielectric. To maximize the energy density of a capacitor, it is desirable to use lightweight materials with a high dielectric constant, e.g., 3-15, and a very high breakdown strength, e.g., 7-16 kv/mil. Because the storage capacity has a squared dependence on the electric field, the breakdown strength of the dielectric is of primary importance in high energy storage of the dielectric in this application. Polymers are generally of low density (approximately 1 to 2 grams/cm.sup.3), and tend to have high breakdown strengths, which offset their low dielectric constant values in capacitor applications.
For most materials, the dielectric breakdown values are in the 10 Kv/mil range, which can translate to potential energy densities on the order of 1 kilojoule/kilogram.
Some of the energy stored in a capacitor will be dissipated as heat due to losses in the electrode and the dielectric medium. The dissipation factor is a measure of the amount of stored energy converted to heat in the dielectric medium. In high burst mode operation, the capacitor operates essentially under adiabatic conditions, with negligible heat loss or dissipation to the surroundings. All the energy lost in the capacitor goes towards heating the capacitor. The temperature rise due to dielectric heating, under adiabatic conditions is given by: EQU VT .alpha. E.times.DF.times.P/M Cp
where E is the energy stored per pulse, DF is the dissipation factor, P is the repetition rate and M and Cp are the mass and specific heat of the capacitor, respectively. Since E/M is the energy density of the capacitor, this equation may be written as: EQU VT .alpha. Ed.times.DF.times.P/M Cp
Clearly to achieve a high energy density and high repetition rate with a low temperature increase, it is desirable to use a dielectric material which has a low dissipation factor, high breakdown strength, high thermal stability, and a low coefficient of thermal expansion. It is desirable to have such a dielectric which enables a capacitor to exhibit stable electrical properties over a wide range of temperatures and frequencies. Currently available capacitors are deficient both in terms of energy storage and repetition rate, as well as operating temperature range due to deficiencies in the available dielectric materials.